Seems I need to take my own advice! Just took my M4 apart to see why it had jammed....piston head had come unscrewed. D'OH! Off to buy more threadlock.
Only bearing piston heads come apart it seems, either loads of thread glue, spring washer, nylon locking nut, or add a thin neoprene washer behind piston to act as a spring washer to stop that mofo coming apart.